Joh 13:34-35 A new commandment I give to you, that you love one another: just as I have loved you, you also are to love one another. (35) By this all people will know that you are my disciples, if you have love for one another.”
1Co 13:1-2 If I speak in the tongues of men and of angels, but have not love, I am a noisy gong or a clanging cymbal. (2) And if I have prophetic powers, and understand all mysteries and all knowledge, and if I have all faith, so as to remove mountains, but have not love, I am nothing.
Every real Christian, that is people who have truly been born again, have been taught to love one another by the Spirit of Christ in them. This is our new creation nature in Jesus. His Law is written on our hearts so that it is part of who the new man is. And this is why Jesus and Paul and other biblical authors tell us that love, the real love of Christ, is THE evidence that we know Him. Specifically, this love is love for Christ and love for Christ’s people. Certainly we show love toward unbelievers as well in various ways but the real test is a person’s response to the Lord and to His people. The “nicest” pagan to be found will eventually cease to be “nice” if pressed in regard to his thoughts on Jesus.
Paul tells the Corinthians that a person can wear the finest most convincing “holy” disguise all they like, if they do not love then they are phonies.